Galileans to Scale

Interiors of the Galileans

Overview of Surfaces

Outer Galilean Moons

Callisto: • Moderately dark

surface• Heavily cratered• No magnetic field• ~1/4 of interior is

ice (by mass)

Ganymede:• Moderately bright

surface• Weak magnetic

field and evidence for a denser core

• Perhaps melted in past to allow for differentiation

Callisto Terrain

Ganymede Terrain

Inner Galilean Moons

Europa:• Bright surface• Absence of craters –

young surface• Possible mag. Field

and denser core• Surface ice is a thin

layer (100’s of meters) floating on H2O ocean (!) – requires a source of heating

Io:• Rocky interior• Orbits inside Jupiter’s

magnetosphere• Io is tidally heated and

volcanically active (predicted in 1979 and discovered by Voyager 1 in same year)

• Surrounded by a yellow Na-cloud

Europa Terrain

Crater on Europa

Close-up on Europa

Subsurface Model

IO: Example Volcanic Activity

Volcanic Eruption at Io: Before and After

Io’s Sodium Cloud

Sulphur Ring of Io

Titan

• 1944 – Kuiper discovers a thick atmosphere• Atmosphere is a reddish, featureless haze; mainly

N2 with 10% CH4 and smog:

• Psurf=1.6PE and Tair=93 K (-292oF)

• Only satellite with a thick atm.:

– CH4 rain, snow, and ice?

– Pools of liquid N2 and CH4

• 2004 – Cassini probe called Huygens parachuted into Titan’s atmosphere

Titan to Scale

Impression of View of Saturn from above Titan

Surface of Titan

Earth-Titan Comparison

The Active Atmosphere of Titan

Huygens Probe at Titan

Cold Geysers at Enceladus

An Ocean Below Enceladus’ Icy Crust?• NASA’s Cassini spacecraft

has observed plumes of material escaping from Saturn’s small icy moon, Enceladus

• The plume is mostly water vapor, with tiny ice particles and other gaseous molecules mixed in (e.g. CO2, CH4, C2H6)

• The plume supplies ice particles to one of Saturn’s rings

• Some ice particles contain salt, which may indicate they originate in an ocean deep below the icy crust

Image mosaic of Enceladus taken by Cassini, showing individual plumes of gas and ice escaping from the surface. The plumes extend 100’s of km into space from the ~500 km diameter moon.

• Plumes may be material escaping through surface cracks from an internal salty ocean or lake

• Alternatively, ice along cracks may sublime or melt, followed by escape of water vapor and icy particles

• Many scientists find the salty ocean model most convincing, but others favor combinations of alternative explanations

What Process Creates the Plume?

Left: Enceladus may have a salty subsurface ocean that releases material to space through cracks in the moon’s icy shell. Right: The walls of icy cracks in the surface may melt or sublime, venting gas and icy particles to space.

The Big Picture

• Enceladus is surprisingly active for such a small body - likely a consequence of tidal heating

• Future flybys of Enceladus by Cassini may help to resolve whether Enceladus joins the growing “club” of solar system bodies believed to have oceans

• If Enceladus has an ocean, then it contains all of the ‘‘ingredients’’ known to be important for life: liquid water, molecular building blocks, and energy

Image of Enceladus showing the ‘tiger stripes’ region in the southern hemisphere, where the plumes originate

Tiger stripes

Mimas

The Impact at MimasDiameter ~ 390 kmHuge crater from impact, almost big enough to shatter this

moon

Estimate:

Binding Energy of Mimas

Kinetic Energy of Collider

Mass and Size of Collider

m v M

D

H2O

Moons of UranusUmbriel and Oberon• Heavily cratered• Large cracks

Ariel and Titania• Lighter cratering• Resurfacing by flows of volcanic water (!)• Some regions of Ariel are featureless - very recent!• System of deep cracks (tidal heating effects?)

Miranda• Rolling and cratered terrain• Some fractures• 20 km cliff - takes 10 min to fall (in contrast to 1 min as at Earth)

Moons of Uranus

Miranda

Triton: Geyser Plumes

Triton• Triton is the only large moon with a retrograde

orbit (possibly a captured body?)• Tidal bulges on Neptune lag instead of lead Triton

Orbit is degradingWill eventually shatter as nears Roche Limit (in

~109 years to make a ring to rival Saturn’s)• Composition: 75% rock, 25% ice• Atm.: thin N2

• Polar Capts: N and CH4 ices• Evidence for resurfacing and internal activity,

even geyser eruptions of N gas, leading to sooty plumes

The Tides at Triton